Telephone connections which are set up via data networks are becoming increasingly important. In the field of telecommunications, there are various options for transmitting voice data in a packet data format via a data network (examples are VoIP, VoATM, VoDSL, VoCM). VoIP (voice over IP) denotes the setup of a voice connection via the Internet. Voice connections are set up by the transmitter via an analog line or an ISDN line.
FIG. 1 shows subscriber linecards for connecting a multiplicity of telephone subscribers to a data network, particularly to the Internet. Each subscriber linecard contains a network processor which is connected to the data network via a network interface (backplane). The network processor or main processor on the subscriber linecard interchanges data packets with the data network, these normally being Ethernet data packets or ATM cells.
In the case of a conventional subscriber linecard, the network processor is connected to various signal processors by means of a microcontroller interface bus. The voice signal processors are normally digital signal processors (DSP). The digital signal processors (DSP) generate signaling systems, compensate for echo signals and perform data compression operations. Each DSP processor is for its part connected to a plurality of subscriber line ports which normally have an SLIC circuit (SLIC: Subscriber Line Interface Circuit) and a CODEC for connecting a subscriber terminal. The SLIC circuit can have an analog telephone terminal connected to it directly. A splitter may additionally be used to connect a data modem. The analog voice signals received from the analog telephone terminal are converted into digital voice data by an analog/digital converter provided in the subscriber port. Conversely, the voice data provided for the subscriber by the DSP processor are converted into an analog voice signal by a digital/analog converter within the subscriber port and are sent to the analog telephone terminal by the SLIC circuit.
The number of digital signal processors (DSP) on a subscriber linecard differs, in the same way as the number of subscriber line ports provided for each digital processor. In the case of conventional subscriber linecards, the number of subscriber line ports is approximately 4-64 subscriber line ports on one subscriber linecard.
Each subscriber linecard is provided with precisely one network processor or controller which is normally connected to a plurality of signal processors (DSP) by means of the common microcontroller interface bus. In the case of conventional Voice over IP subscriber linecards, the network processor or controller takes from the signaling in order to set up a telephone connection between subscriber terminals, i.e. analog telephone sets, which are connected to the associated SLIC circuit. In this case, the signaling is performed using signaling protocols such as SIP, MGCP, H323.
FIG. 2 is used to explain setup of a telephone connection via a data network based on the prior art. If the subscriber on the telephone terminal A wishes to set up a telephone connection to the telephone terminal B, then the digital signal processor DSPA generates a call-connected signal when the handset of telephone terminal A has been identified as having been lifted, and first of all sends the identified digits of B's telephone number which have been dialed to its own network processor A. The network processor A transmits the identified telephone number to a gatekeeper via the data network. The gatekeeper uses a stored table to translate the dialed telephone number of the telephone terminal B into an associated IP address and returns this address to the querying network processor A. At that point, the actual connection setup takes place by virtue of the network processor A sending to the identified IP address of the network processor B a request to set up a telephone connection. The telephone connection between the network processor A and the network processor B is set up using the known signaling protocols such SIP, MGCP, H323. The called subscriber's digital signal processor B uses the associated port of subscriber B to generate a ringtone for the telephone terminal B. When the receiver of the analog telephone B has been lifted, the telephone connection between the two subscribers A, B has been set up.
The voice data are interchanged between the two subscribers A, B via the data network using Voice over IP data packets. FIG. 3 shows the structure of a Voice over IP data packet based on the prior art. An IP data field contains the network addresses of the respective transmitting network processor, for example the network processor A, and the network address of the received network processor, for example the network address of the network processor B. Upon a telephone call, Voice Over IP data packets are interchanged between the two subscribers in both directions simultaneously via the data network. The IP data packet is also provided with a UDP data field (UDP: User Datagram Protocol) and an RTP (Real Time Protocol) data field. The voice data form the payload of the data packet. The length of the data packets is configurable, the duration of the data packet typically being between 5 and 30 ms. The length of the data packets is limited, since the voice data have to be interchanged between the two subscribers in real time, i.e. the time delay must not be excessively long so that it is not perceived as a disruption by the two subscribers involved in the call.
The subscriber linecard based on the prior art which is shown in FIG. 1 has the following drawbacks: The microcontroller interface bus which connects the digital signal processors DSP to the network processor is used to transmit both the signaling data for telephone connections between the telephone terminals and the voice data for the telephone connections which have already been set up. The greater the number of voice signal processors (DSP) which can be integrated on a subscriber linecard and the number of subscriber line ports connected thereto, the lower the hardware costs per subscriber, since the network processor and the network processor interface and also other integrated circuits on the subscriber linecard, such as SDRAM, flash memory, PowerIC, need to be fitted on the subscriber linecard regardless of the number of subscriber line ports provided. As the number of subscriber line ports on the subscriber linecard increases, the data throughput rate on the microcontroller interface bus increases greatly, since the microcontroller interface bus is used to interchange both the signaling data for telephone connections which are to be set up and the voice data for already existing telephone connections between the digital signal processors and the network processor. The microcontroller interface bus therefore forms a bottleneck which severely limits the number of subscriber line ports which can be provided on a subscriber linecard.
FIG. 4 serves to explain this problem scenario. The data throughput rate (DS) and the time delay (V) are plotted schematically on the basis of the number of subscriber line ports provided on the subscriber linecard. The maximum data throughput rate (DSmax) is prescribed by the bus width of the microcontroller interface bus, by the bus clock frequency and by the processing speed of the network processor. The maximum permissible time delay (Vmax) is obtained from the real-time condition of a telephone call and corresponds approximately to the length of a Voice over IP data packet, typically between 5 and 30 ms. As the number of active subscriber line ports increases, the data throughput DS and the time delay (V) increase, as shown in FIG. 4. In this context, the time delay arrives relatively soon at the maximum permissible delay time Vmax and thus limits the number of maximum permissible subscriber line ports on the subscriber linecard. If the delay time Vmax is exceeded, this results in a loss of data, since the network processor is not delivering to the other subscriber or fetching therefrom within the maximum permissible delay time as a result of the Voice over IP data packet. The greatly limited number of permissible number of subscriber line ports on the subscriber linecard means that the costs for the network connection increase per subscriber.
A further drawback of the subscriber linecard based on the prior art which is shown in FIG. 1 is that there is no separation between the control signals for setting up a data connection and the actual flow of data. This means that the software for the network processor is relatively complicated and it is barely possible to make deterministic predictions. The network processor can be produced only with some technical complexity on account of the many position of signaling and voice data. In this case, identifying whether the data are voice data or signaling data requires a particular identification time, which increases the signal delay time overall.
A further drawback of the conventional subscriber linecards based on the prior art can be seen in FIG. 5. When the subscriber linecard has a modular design with a plurality of subscriber modules and a network module, it is not possible to exchange a subscriber module in the course of operation. The subscriber modules are connected to the network processor by means of connectors (ST) via a common microcontroller interface bus. It is not possible to connect or disconnect a subscriber module to the microprocessor interface bus in the course of operation, i.e. there is no hot plug capability.